The present invention relates to a system and method for providing multicharge ignition, and more specifically, to a method and system adapted to trigger at least some of the multicharge events of the system and method in a current-dependent manner and further adapted to terminate the sequence of recharging and partially discharging the inductive energy storage device of the ignition system based on a timing signal and without requiring other signals indicative of crank angle.
Generally, a repetitive spark distributorless ignition system stops ignition current before the complete discharge of magnetic energy in the ignition coil supplying the spark plug. During the stoppage, the ignition coil is recharged so an additional spark can be applied to the spark plug. The present invention relates to a system and method for igniting a combustible gaseous mixture, particularly a mixture of gasoline vapor and air in the combustion chamber of an internal combustion engine utilizing a spark plug.
Ignition of a fuel-air mixture in the combustion chamber of an internal combustion engine (ICE) is done by a spark plug in which a high-voltage spark, for example generated by discharge of a capacitor or coil, is caused to discharge across a firing or spark gap of the spark plug. The capacitor, or another energy storage device such as an ignition coil itself, is charged with energy and, at a predetermined time instant which may be controlled by a computer, the capacitor or other energy storage device discharges causing the spark to flash over at the spark gap. The spark gap ignites the combustible mixture within the combustion chamber of the ICE.
Timing of the spark in relation to the combustible charge, and the position of a piston in the ICE, usually taken with reference to the top dead-center (TDC) position of the piston, is important. The spark flash over usually is caused to occur at a predetermined time instant in advance of the TDC position of the piston so that the mixture will burn, and give off energy just at and after the piston has reached TDC position. To obtain maximum efficiency from the burning operation, it is important that the mixture should burn as rapidly as possible within the combustion chamber, and that a frontal zone of combustion, or flaming, of the combustible mixture propagates as rapidly as possible.
The electrical discharge which occurs at the spark gap of the spark plug under control of the associated ignition system is, unfortunately, not a clearly analyzable occurrence or event as, for example, an electrical square-wave pulse or the like which controls the discharge. Rudolf Maly of the Institut fur Physikalische Elektronik, Universitat Stuttgart, has suggested in numerous papers that as the spark forms, three phases can be distinguished, namely, (1) the breakdown phase, (2) the arcing phase, and (3) the glow phase.
The energy transferred in the various phases differs greatly. The formation of the respective phases depends to some extent on the geometry of the ignition electrodes, as well as on the associated circuitry connected thereto. If the ignition system provides a high-voltage pulse to the ignition electrodes, then, first, after the breakdown voltage has been exceeded, an electrically conductive plasma path will result. The currents which flow through the path between the electrodes may be very high. This occurs during phase (1), that is, the breakdown phase as the voltage falls from very high voltages (kilovolts) to voltages less than 10% of the peak.
The next phase is the arcing phase, the formation and course of which depends to some extent on the circuitry with which the spark plug is associated. The arcing phase causes current to flow in the previously generated plasma path. The voltage between the electrodes may be comparatively low or the current which flows at the beginning of the second, or arcing phase may be high. When the current during the arcing phase drops below a transition threshold, the arc will degenerate into a third, or glow phase which usually follows. The current during the third or glow phase continues to supply thermal energy to the media in the gap although much is lost to the electrodes during the relatively long period of time. During the glow phase, the voltage is above the value of the arcing phase voltage.
The spark plug is stressed differentially during the respective phases. In the breakdown phase, the heat loading on the spark plug is low. In the arcing phase, the heat loading is high, and heat which is applied to the ignition electrodes of the spark plug leads to the well known erosion and deterioration of the spark plug. Relatively little erosion takes place during the glow discharge because of the low current densities and currents (&lt;100 ma) that can be sustained.
The loading conditions applied to an Otto-type ICE result in different conditions of combustible mixtures in the combustion chamber. Upon full load operation, the mixture is rich and the degree of fill of the combustion chamber is high. Igniting such a mixture does not pose any significant problems. An accelerated transfer of energy is not even necessarily desired. If the ICE, however, operates at low loading, or under idling condition or, even under engine braking conditions, the temperature within the combustion chamber drops rapidly and the pressure also drops. The mixture is lean, and the degree of fill of the combustion chamber of the ICE is low. Non-homogenates of the mixture occur, and consequently, ignition of the already lean, and possibly non-homogenous and insufficiently filled, mixture may cause difficulties.
Ignition systems are known which provide a succession of spark breakdowns in order to ensure ignition of the combustible mixture in an ICE. For example, it is known to sense the composition of the combustible fuel-air mixture, and to control the number of spark flash-overs, or breakdowns at the sparking electrodes of the spark plug as a function of the ratio of fuel to air in the combustible fuel-air mixture.
U.S. Pat. No. 4,653,459 to Herden teaches engine control using the relationship of the number of spark breakdowns to the fuel-air mixture composition being supplied of the engine. However, specially constructed spark plugs are required to enhance the breakdown phase. Furthermore, the higher energy impulses of these breakdown sparks may lead to undesirable RFI (radio frequency interference) emissions.
To avoid having to reconfigure the ignition components, U.S. Pat. No. 5,014,676 to Boyer suggests the desirability of using conventional inductive discharge hardware, preferably in a distributorless configuration, with repetitive firing, and further suggests communicating the ON/OFF control for this mode from a main engine control computer. According to the '676 patent, by truncating the length of each glow discharge to recover energy which otherwise would be lost to the spark plug electrodes and providing a number of fresh ignition sources in a turbulent mixture by repetitively firing the same spark plug gap, there exists a higher probability of igniting a lean mixture.
While the arrangement disclosed in the '676 patent is acceptable in many situations, it does not adequately compensate for actual variations in the conditions within the combustion chamber after the first spark. Once the '676 arrangement determines, based on the operating conditions of the engine, that sparking will be provided repetitively, the events that trigger each application of energy which is intended to generate one of the sparks are primarily time-based events. That is, each attempt to generate a spark in the repetitive sequence is triggered and terminated at specified times.
While the specified times are different from one attempt to the next, they are pre-set and do not change to compensate for actual variations in the amount of energy required to recharge the energy storage device (e.g., the ignition coil) for the next application of a spark. Nor do the pre-set time values change to compensate for actual variations in the amount of energy dissipated by each spark subsequent to the first. When these actual variations are significant, which is not uncommon due to variations in the conditions within the combustion chamber, the arrangement disclosed in the '676 patent provides less than ideal firing characteristics.
The variations in conditions within the combustion chamber (e.g., whether there is a high-flow condition or a low-flow condition in the combustion chamber) can cause the amount of energy dissipated by a sparking event subsequent to the initial spark to vary by as much as one order of magnitude. In low flow conditions, for example, it may take as little as 200-300 volts to sustain a spark after the initial spark. In particular, the medium between the electrodes of the spark plug remains ionized and therefore facilitates restriking of the spark plug. Under high flow conditions, by contrast, it may take 2,000 volts to sustain the same spark in the sequence because of the lack of ionization between the electrodes of the spark plug. There consequently can be a 10:1 variation in the amount of energy dissipated and thus in the amount of energy required by the coil to ensure that a spark is sustained. Such large variations mean that if the discharge trigger time is pre-set based on the erroneous assumption that the combustion chamber conditions will require only a small amount of energy to ignite the spark, the amount of time allocated for recharging may be too short to sustain the desired spark (e.g., in high flow conditions). Conversely, if the discharge trigger time is pre-set based on the opposite erroneous assumption, namely, that the combustion chamber conditions will require a large amount of energy to ignite the spark, then the time allocated to recharging may be longer than is necessary, thereby unduly lengthening the time between successive sparks and/or overcharging the coil. In either case, the ignition system would provide less than ideal performance.
Even if the pre-set times are determined based on the assumption that the conditions within the combustion chamber will remain substantially mid-range between those requiring a large amount of energy and those requiring little energy, the magnitude of possible variations in energy requirements (i.e., the aforementioned 10:1 ratio) prevents that approach from completely eliminating the potential for inadequate performance.
There is consequently a need in the art for a multicharge ignition system capable of providing the advantages associated with repetitive spark generation, while adequately compensating for variations in dissipation and recharge energy from one spark event to the next in each repetitive spark generation sequence. In this regard, there is a need in the art for a multicharge ignition system in which the discharge events are triggered based on the amount of energy stored in the coil of the ignition system.
While U.S. Pat. No. 5,462,036 to Kugler et al. does provide discharge events that are triggered based on the amount of current in a primary winding, the device disclosed by Kugler et al. requires more than one input signal (e.g., speed of rotation n, pressure p, supply voltage Up, temperature T, and the like). These signals are used by the Kugler et al. device to determine, among other things, the ignition time ZZP. Since the Kugler et al. device is not responsive to a single timing signal (e.g., an EST signal) from a PTCU, but rather a plurality of input signals, it generally is employed as a replacement for existing PTCUs.
Replacement or modification of existing PTCUs, however, is not necessarily desirable or practical. Manufacturing of existing PTCU's has been substantially refined over the many manufacturing runs of the PTCUs. The use of existing PTCU's also tends to minimize tool-up time and production costs. In addition, since existing PTCUs have been used and tested in actual vehicles and have been refined based on the results of such use over significant periods of time, it is generally desirable to take advantage of their proven reliability by providing an ignition system that uses existing PTCUs and adds little, if anything, more than what is necessary to enable existing PTCU's to provide multicharge ignition. In this regard, there is generally a need for a multicharge ignition system and method adapted to terminate the sequence of recharging and partially discharging the inductive energy storage device based on the timing signal (e.g., the EST signal) from an existing PTCU. Since manufacturing expedients are achieved by minimizing the inputs to any additional multicharge circuitry, a need exists for multicharge ignition systems and methods that are capable of implementation without requiring input signals other than the timing signal (e.g., without requiring signals indicative of crank angle, for example).